To print an image, a print engine processor, referred to herein as a raster image processor, converts the image in a page description language or vector graphics format to a bit mapped image indicating a value to print at each pixel of the image. Each bit representing a pixel that is “on” is converted to an electronic pulse. The electronic pulses generated from the raster pel data at which to deposit toner turns the laser beam on to positively charge the surface of a rotating drum, which is a photo-conducting cartridge, that has a coating capable of holding an electrostatic charge. The laser beam turns on and off to beam charges at pixel areas on a scan line across the drum that will ultimately represent the output image. After the laser beam charges all pels on the scan line indicated in the raster data, the drum rotates so the laser beam can place charges on the next scan line. The drum with the electrostatic positive charges then passes over negatively charged toner. The negatively charged toner is then attracted to the positive charged areas of the drum that form the image. The paper, which is negatively charged, passes over the roller drum and attracts the toner as the areas of the roller drum with the toner are positively charged to transfer the toner forming the image from the roller drum to the paper.
Note that the terms “pixel” and “pel” are used throughout this specification in an interchangeable manner. Each term refers to one dot or point of data which make up the complete image. Also, the above discussion describes the drum, the toner and other components as having certain charges allowing the toner to ultimately be drawn to the paper. Similar printing systems are well known to those skilled in the art which utilize components having the opposite charges, providing the same end result. This and other variations in the system of producing printed output may be made without departing from the scope of the present invention.
Many modern laser printers filter the bit map images using a look-up table to alter the pulses generated for each pixel to accomplish certain filtering results. For instance, filters can be used to provide an economy mode where toner is reduced, remove jagged edges, improve image quality, improve print quality using known techniques referred to as ‘Print Quality Enhancement’ (PQE) or reduce the density of images. The subject matter of this invention is primarily concerned with filtering for PQE purposes. Typically, the laser printer will gather an area of data and replace either one or all the pulse values for the pixels based on the gathered area of pixel data matching a value in the look-up table. Such look-up tables modify the pixel output by altering the pulse normally used for an “on” pixel value with a pulse width modulator to shorten the pulse width to reduce the electric charge the laser beam places on the drum. This technique of shortening the pulse width for a pixel is referred to as ‘sub-pulse modulation.’ By shortening the pulse width associated with a pixel of data to a ‘sub-pulse’, the resulting image will not cover the entire pixel area. For instance, if the pulse width is modulated to one-half a full width, the image in that pixel area will only cover one-half the area of the pixel.
The look up tables used for sub-pulse modulation may also contain alignment information for aligning the sub-pulse within the pel region for improved print quality. The techniques used to perform sub-pulse modulation and many varying rules for aligning the created sub-pulses to attain PQE are well known to those skilled in the relevant arts. Pels printed using less than the full width pulse for PQE purposes may be referred to as “gray-scale” pels as they are printed somewhere between white and black. Techniques other than look-up tables may also be utilized for PQE purposes as long as such techniques provide for pre-calculation of pulse width values to be associated with predicted patterns of pel data.
A system utilizing sub-pulse modulation for PQE purposes must feed a pulse width modulator with information indicating the width and alignment of the pulse to create for each pixel. This means that the pulse width modulator must be able to accept and process the input and create and deliver the required pulse width to the laser within the time it takes the printer to print one pel. As modern, high-function printers continue to operate at faster and faster speeds, this becomes more difficult. The latest printers are capable of operating at speeds of 100 MHz or greater, which leads to a “Pel Time”, or the time needed by the printer to process one pixel, of 10 ns or less. Another way to measure the speed of a printer is called the “Video Data Rate”, which is the number of pels written on the drum by the printhead per second. As printer speeds continue to increase, it is becoming more and more difficult for existing and future pulse width modulators to function properly. Increases in speed are more difficult to achieve in the pulse width modulators because some minimum time is required to reset the pulse width modulator after each pulse.
For the above reasons, there is a need in the art for an improved technique for processing pels of print output for PQE in order to allow current and future pulse width modulators to keep up with the increasing printer speeds without reducing print image quality.